Recently, strict control and regulation of vehicle emissions, in many countries including the USA and Europe, has accelerated development of electric vehicles (EVs) and hybrid electric vehicles (HEVs) using internal combustion engines and batteries as power sources, thereby partially entering commercialization.
Currently, batteries that can be utilized in EVs or HEVs are high power batteries and for example, Ni-MH secondary batteries, safety of which was verified, are commercially available. In addition, development of lithium secondary batteries having output density and energy density superior to Ni-MH secondary batteries is also actively underway.
However, lithium secondary batteries for use in EVs require not only high energy density and capability to exert large power output within a short period of time, but also a long-term service life of more than 10 years even under severe conditions in which high current charge/discharge cycles are repeated within a short term, thus necessitating remarkably superior safety and long-term service life properties compared to conventional small-size lithium secondary batteries.
Lithium ion batteries that have been used in conventional small size batteries generally employ a layered structure of lithium cobalt composite oxide as a cathode material and graphite-based material as an anode material. However, the main constitutional element of the lithium cobalt composite oxide, cobalt, is very expensive and is not suitable for use in electric vehicles due to safety concerns. Therefore, as the cathode material of lithium ion batteries for EVs, lithium manganese composite oxide having a spinel structure made up of manganese is ideal in terms of both cost and safety. However, the lithium manganese composite oxide, upon high-temperature and high current charge/discharge, undergoes elution of manganese ions into an electrolyte due to the influence of the electrolyte, thus resulting in degradation of battery properties and performance. Thus, there is a need for measures to prevent such problems. In addition, the lithium manganese composite oxide has drawbacks such low charge density as compared to conventional lithium cobalt composite oxide or lithium nickel composite oxide. Thus, there is a limit to charge density of the battery and in order to enter practical use as the powder source of EVs, HEVs, etc., specific designs of the battery to overcome such disadvantages should be effected together.
In order to alleviate the above-mentioned respective disadvantages, various studies and attempts to prepare electrodes using a mixed cathode active material have been made. For example, Japanese Patent Publication Laid-open Nos. 2002-110253 and 2003-168430 disclose techniques utilizing a mixture of lithium manganese oxide and/or lithium cobalt oxide, and lithium nickel-manganese-cobalt composite oxide (B) to enhance recovery output. These techniques, however, suffer from problems associated with inferior cycle life span of the lithium manganese oxide and limited improvement of safety.